Well, Google wouldn't replace their login process because they are the BrowserID primary. If you have a gmail account, BrowserID expects you to be logged in with it, which essentially means you're logged in to Google. (They could add it for non-gmail Google accounts, I guess.)

But Facebook could benefit from this. Maybe not at this early stage, but the way you log into Facebook is using your email account. That's exactly the step BrowserID wants to make easier.